For analyzing chemical substances, foods, agricultural products and so on, near-infrared absorptiometric analysis has been in practical application. Using the resulting analytical data, control of production process for such substances may be realized. In the field of chemical industry, it has been proposed to realize control of operation of a production plant for chemical product by using data of near-infrared absorptiometric analyses of the raw materials, solvents, moisture content, intermediate products, final product, by-products and so on. In conventional near-infrared absorptiometric analysis, a near-infrared absorbance spectrum in a specific range (in the following, denoted sometimes as near-infrared spectrum) is taken and, based on a specific combination of absorbances at specific wave lengths included in this spectrum, the components, characteristic properties and so on to be determined are calculated by having resort to a preliminarily prepared calibration curve, in order to derive analysis values (predicted values).
In one generalized example of practicing the near-infrared absorptiometric analysis, a correlation equation (calibration curve) is prepared by absorbance spectra in a wave length range exhibiting a correlation obtained by near-infrared absorptiometry and analytical results obtained by conventional analysis practice, in order to presume the prospective quantitative analysis value. This analysis value is only a predicted value calculated using the calibration curve.
A near-infrared absorptiometry accompanies an inherent defective characteristic feature that there occurs a shift of the spectrum due to influences by, for example, moisture content and temperature of the objective material. This spectrum shift may behave as if there is an alteration in the concentration of the component or in another material property of the material to be observed, even if there is in fact no such alteration. When operation of a plant is carried out based on such false results, the resulting product will have an extra-rated quality.
By a near-infrared absorptiometry, a definite absorbance spectrum can be obtained steadily for a specific component under a definite observation condition and with specific material properties, while the absorbance spectrum may subject to deviation in the height or position of absorbance peak due to alteration in the condition, such as concentration, particle size and temperature, or may vary due to interference with the absorption peaks for co-existing extraneous components. From a near-infrared absorbance spectrum, which includes, as mentioned above, information for a plurality of constituent components, a calibration curve (correlation equation) for each component is prepared by means of a statistic technique, on the basis of which analysis is attained.
For the preparation of the calibration curve, samples each having a definite composition and definite characteristic features are taken and are subjected to conventional chemical analysis and to a near-infrared absorptiometric analysis, whereupon a correlation equation is derived by means of a statistic technique, such as a multiple linear regression analysis (MLR) or a partial least-squares method (PLS). A near-infrared absorbance spectrum includes a number of absorbance peaks, so that use of too many descriptive variables (assigned wave lengths) may result in reduction in the reliability due to overfitting of calibration curves. For this reason, there are used in general 2 to 5 descriptive variables for MLR and about ten descriptive variables for PLS.
It is a common practice for detecting the component concentrations and material properties using a wave length range of near-infrared region from 800 to 2,500 nm, that predicted values are estimated by selecting near-infrared spectral wave lengths each having a correlation with the results determined using conventional analysis apparatus or conventional material property observing apparatus under preparation of a correlation equation to obtain a predicted value of near-infrared analysis. However, the so-estimated predicted values represent only informations derived using the calibration curve prepared from values for limited wave lengths in numbers of 2–5 or about 10 selected within the wide near-infrared region of from 800 nm to 2,500 nm. Therefore, it is difficult by such a common practice to grasp the entire aspects or a delicate variation of a sample (or a product) and such a practice is not effective, in particular, in the case where preparation of calibration curve is not possible. Namely, it is difficult to prepare a calibration curve in the following cases:                (1) Variation of the parameter to be detected, such as concentration or material properties, is little.        (2) Temporal change of the sample occurs.        
The object of the present invention is to provide a method of controlling production process capable of controlling the production process steps by a simple manner at a higher accuracy based on the result of analysis performed by a near-infrared absorptiometry without using calibration curve.